1. Field:
This invention relates to the transport of finely divided, particulate material from a stockpile at a controlled feed rate. It is specifically directed to methods and apparatus for transporting particulate material at precisely controlled, low feed rates. Specific applications of the apparatus of this invention include the introduction of catalysts or reactants to reaction zones and the supply of metal powder to plasma flame spraying apparatus.
2. State of the Art:
The pneumatic conveying of finely divided, particulate material is conventional. The venturi principle is employed in various commercial equipment for drawing particulate material from a reservoir into a moving stream of gas or liquid. Such techniques have not heretofore been successfully applied to the uniform transport of particulate materials at low rates, however.
Heretofore, for applications requiring uniform feeding of a powdered material, e.g., to a plasma flame for plasma flame spraying of a substrate, various types of mechanical feeders have been relied upon. At very low rates, mechanical feeders become erratic, however. Limitations imposed by the mechanics of such feeders make it impractical to feed powders from a stock at rates much below about a pound per hour. Many potential applications exist for well-controlled, uniform, lower feed rates, but it has not heretofore been practical to achieve them.
U.S. Pat. No. 3,387,110 describes an apparatus for uniform feeding of powder into a plasma spray gun. The apparatus comprises a container with a gas inlet channel opening into its bottom and a powder suction channel at its top, directly opposite the gas inlet channel. A turbulence and suction tube extends down into the container from closed communication with the powder suction channel. In operation, a working gas is introduced through the inlet channel into the container to agitate the powder, the gas causes the powder supply in the container to whirl about within the container and the stirred-up powder is drawn off by suction from the upper part of the container. Proper operation of the device requires the inlet channel to be covered by powder stored within the container. Accordingly, the suction and turbulence tube must terminate a sufficient distance above the bottom of the container to permit powder to flow over the inlet channel. The rate of withdrawal of powder from the container is dependent upon the suction applied at the top of the container and is thus influenced by pressure changes in the plasma. Accordingly, it is not practical to rely upon such a device to deliver particulate matter at a controlled, steady rate to apparatus which experiences periodic or erratic pressure changes.